The spleen is the largest secondary immune system organ in man. Individuals who are asplenic die at an increased rate from overwhelming sepsis. They are also at greater risk of autoimmune disease, some cancers and ischemic cardiac disease. Enhanced mortality is attributed to lack of phagocytes sequestered in spleen that efficiently eliminate appropriate targets, though related cells can be found at other sites. Although the overall organization of splenic white pulp is similar to that of lymph nodes, spleen is not connected to the lymphatic system. Instead, pathogens, RBCs, apoptotic/necrotic, altered and tumor cells all are delivered to and leave the spleen in blood. In contrast to white pulp, splenic red pulp serves as both a reservoir and a filter that determines whether cells coursing therein are retained, destroyed or returned to the circulation. The human venous sinus-lining cell, known as the littoral cell (LC), comprises ~30% of red pulp. Increasing evidence indicates these highly specialized splenocytes are the major determinant of whether RBCs and others are destroyed, retained or returned to the circulation, though the mechanisms that regulate their function are entirely unknown. Others and we showed that LCs are not a classical endothelial cells as they bear multiple macrophage and smooth muscle-associated antigens, are CD8+ and lack traditional markers of endothelial and also hematopoietic lineage. So, why is so little known about LC function? The acquisition, purification and culture of LCs have historically proved very difficult. Based on our recent success in obtaining fresh spleen as well as identifying markers that distinguish LCs from other splenocytes, we are now prepared to further isolate and profile LC gene/protein expression (AIM I). Although cells such as LCs that form specialized structures within complex organs are often hard to propagate ex vivo, overcoming these limitations would represent a significant advance in understanding LC function(s). We have assembled a group of expert endothelial cell biologist-immunologists, and pathologists who will provide us with in depth guidance as we develop diverse LC culture systems (AIM II) that are both physiologically relevant and tractable. These methods will enable discovery of the mechanisms that underlie LC function and lead to development of therapeutics able to modulate splenic filtration in different disease states (e.g. sepsis, malaria, hemoglobinopathies and others).